The present invention is in the field of optics for implementing a depolarizer and a polarization transformer by means of a corner-cube.
Random polarization changes occur in fiber-optic communication systems. Linearly polarized light launched into a fiber invariably experiences birefringence as it propagates through the fiber. Birefringence transforms linearly polarized light into elliptically polarized light which can compromise performance in communication systems. There are methods for minimizing adverse non-linear polarization effects. One is by means of a standard polarization transformer that converts elliptically polarized light into linearly polarized light. The second is by means of depolarization whereby the polarization states in the Stokes parameters null out. Depolarized light is not affected by birefringence in the fiber and thereby propagates without any polarization perturbations.
Historically polarization transformers have been developed to minimize adverse polarization problems in signal processing. The first transformers were fiber squeezers and mechanically rotated wave plates. More recent transformer developments have been described in U.S. Pat. Nos. 4,996,431 and 5,212,743 (Heismann, Oct. 30, 1990 and May 18, 1993). Heismann's solution is an electrically-controlled integrated optic waveguide device in a LiNbO3 substrate. The device consists of three integrated waveplates with corresponding cascaded electrodes that are controlled by applied drive-voltages transforming the input polarization state into any arbitrary polarization state by TE-TM mode conversion in the LiNbO3 waveguide. Standard polarization transformers based essentially on the concepts of Heismann are commercial available, for example, from Corning and USD Uniphase. Heismann's invention is a relatively expensive item because it requires specialized processing techniques for channel diffusion and lithographic printing of the pattern for the integrated-optics waveguide device. The transformer has a relatively narrow optical wavelength range as well as a limited tunability range. Another transformer has been described by Bismuth in U.S. Pat. No. 6,188,809 (Feb. 13, 2001) that utilizes a 10-cm length electro-optical rod having a 1-mm rectangular cross-section with two sets of electrodes disposed on the four facets of its cross-section. Major problems in Bismuth's invention are 1) the mechanical fragility of the device in implementing a rod having a 100:1 geometrical aspect ratio, and 2) maintaining a stable beam path aligned to the saddle point axis of the electrode fringing fields at the center of the 1-mm cross-section of the rod.
Polarization transformers of the Heismann type can be used as scramblers, otherwise known as pseudo-depolarizers. The scrambler speed has to be fast enough such that the time-averaged depolarization shows no compromising polarization effects in the processed signal at the receiver terminal of the communication link. Other types of depolarization devices are also available. Depolarization can be achieved in a birefringent medium by re-circulating a split-off portion of the output light back into the input of the birefringent medium as described in U.S. Pat. No. 6,421,471 (Shen, Jul. 10, 2002). The output polarization of the light beam averages to zero for a linear series of re-circulating loops, whereby the number of loops in the chain enhances the depolarization factor. The averaging scheme in concept is similar to the scrambler except it is entirely passive without need of electronic drivers. Fiber loops can also be used for depolarizers as described in U.S. Patent Application No. 2003/0063833 (Gonthier, et al., Apr. 3, 2003). Another method is described in U.S. Patent Application No. 2003/0007149 (Yamamoto, Jan. 9, 2003) that uses a series of birefringent plate-pairs that are bonded together such that the optical axis of each pair section is orthogonal to each other. The junction along the optical axis between the pair section is angled geometrically at 45° in order to enhance the mixing or averaging of the polarization states. The depolarization factor can be enhanced by adding more plate-pairs in the beam path resulting in a trade-off in increased throughput loss. Another application of a birefringent plate pair is described in U.S. Pat. No. 6,498,889 (Yao, Dec. 24, 2002) and U.S. Patent Application No. 2003/0112436 (Yao, Jun. 19, 2003), however, unlike Yamamoto's 45° angle junction between the plate-pairs, Yao uses a shallower angle between the plate-pair such that the phase shift across the diameter of the beam is 360°. This allows the polarization states of the beam through the plate-pair to be mapped out in a linear pattern symmetrically about the center line of the beam. Summation of the Stokes polarization parameters null out in the focused spot of the output beam which results in a depolarized beam with only one birefringent plate-pair.
In summary, current depolarization methods for narrow bandwidth lasers require scrambling of the polarization states in the refractive index either by active means in a waveguide medium or by passive means in re-circulating fiber loops or in birefringent plate pairs. Ideally, the aim in depolarization is to create a sufficient number of polarization states such that the sum of the polarization states in the Stokes parameters cancel out to zero. In the above cited patents the output beam from the various depolarizers are temporally-incoherent, thus compromising their use in some systems. It is the intent of the present invention to remedy this situation by providing for a temporally/spatially-coherent depolarizer limited only by the line-width of the laser.